The invention relates to a process for preparing water-expandable polystyrene (WEPS) by polymerizing styrene in aqueous suspension, where the suspended styrene droplets comprise an emulsion of finely dispersed water.
Expandable polystyrene (EPS) beads are usually prepared by polymerizing styrene in aqueous suspension in the presence of a volatile organic blowing agent. The usual blowing agents are hydrocarbons, in particular pentane. In order to protect the environment, pentane emitted during the production and processing of EPS has to be reclaimed. This is complicated and costly. In the longer term, therefore, it would be useful to replace these organic substances with blowing agents which are less hazardous, for example water.
The Eindhoven University 1997 dissertation by J. J. Crevecoeur xe2x80x9cWater Expandable Polystyrenexe2x80x9d describes a process for preparing WEPS by first preparing a fine emulsion of water in styrene with the aid of surface-active substances, polymerizing the styrene to a conversion of 50%, suspending the mixture in water with phase inversion, and finally polymerizing the styrene to completion with the aid of peroxide initiators. The surface-active substances used comprise amphiphilic emulsifiers, e.g. sodium [bis(2-ethylhexyl)sulfosuccinate] or sodium styrenesulfonate, or block copolymers made of polystyrene blocks and of polystyrenesulfonate blocks. All of these substances have both a hydrophilic and a hydrophobic radical and are therefore able to emulsify water in styrene.
A disadvantage of this process is that it is carried out in two stages: water is first emulsified in the styrene/polystyrene mixture and then the organic phase is suspended in water, with phase inversion.
It is an object of the present invention, therefore, to develop a simpler, single-stage process for preparing WEPS.
We have found that this object is achieved by adding, at the start of the suspension polymerization or during its course, from 0.1 to 15% by weight, based on the monomers, of an inorganic solid substance insoluble in water and in styrene and having an average particle size of not more than 100 xcexcm and a density above 1.1 g/cm3.
During the polymerization, gravity and centrifugal forces dictate that the particles of the solid substance continuously penetrate the interface between water phase and styrene phase and while doing this constantly carry adhering water droplets with them into the styrene phase. This is probably the basis for the action of the solids as emulsifying agents.
Preferred solid substance is carbon black with an average particle size of from 10 to 500 nm, depending on the type of carbon black. Even the preferred amounts of from 0.2 to 5% by weight of carbon black are effective. Another suitable substance is graphite with an average particle size (longest lamellar diameter) of from 2 to 20 xcexcm. Active amounts are preferably from 0.4 to 10% by weight.
Other suitable substances are silica gel, silicates, e.g. talc or bentonite, metal oxides, e.g. Al2O3 and TiO2, hydroxides, such as AlO(OH) and Mg(OH)2, and also metal salts, such as CaCO3, Mg3(PO4) and BaSO4.
In the novel suspension polymerization it is preferable for styrene alone to be used as monomer. However, up to 20% of the weight of styrene may have been replaced by other ethylenically unsaturated monomers, such as alkylstyrenes, divinylbenzene, acrylonitrile, 1,1-diphenylethene or xcex1-methylstyrene.
In the suspension polymerization use may be made of the customary auxiliaries, e.g. suspension stabilizers, free-radical initiators, flame retardants, chain transfer agents, expansion aids, nucleating agents and plasticizers. It is advantageous for the suspension stabilizers used to be inorganic Pickering dispersing agents, e.g. magnesium pyrophosphate or tricalcium phosphate, combined with small amounts of alkylsulfonates. Preferred flame retardants are organic bromine compounds, such as hexabromocyclododecane, and the amounts of these added are from 0.1 to 2% by weight, based on the monomers.
It is advantageous to carry out the polymerization in the presence of from 1 to 30% by weight, preferably from 3 to 15% by weight, of polystyrene, and it is appropriate for this to be used as a solution in styrene. It is also possible to use recycled polystyrene material instead of pure polystyrene.
The solid substance is preferably added straight away at the start of the suspension polymerization, but it may also be fed during the course of the polymerization until the conversion is 90%.
It is appropriate for the suspension polymerization to be carried out in two temperature phases, using two peroxide initiators decomposing at different temperatures. The suspension is first heated to 80-90xc2x0 C., whereupon the first peroxide, e.g. dibenzoyl peroxide, decomposes and the polymerization begins. The temperature is then allowed slowly to rise to 100-140xc2x0 C., whereupon the second peroxide, e.g. dicumyl peroxide or di-tert-butyl perbenzoate, decomposes.
The WEPS beads produced during the suspension polymerization comprise, depending on the amount of recycled EPS material used and the content of coating agent, from 2 to 20% by weight, in particular from 5 to 15% by weight, of water. Their particle size is from 0.2 to 5 mm, preferably from 0.5 to 2 mm. They may be foamed using air at from 110 to 140xc2x0 C. or superheated steam, to give foam beads. A particularly elegant foaming process which gives foam beads with a very low bulk density is described in the German Patent Application P 198 12 854.1.
The WEPS foam beads may, like conventional EPS foam beads, be fused to give foam sheets, foam slabs or foam moldings, each of which can be used as an insulating or packaging material.
All percentages in the examples are based on weight.